The invention relates to an apparatus and method for pumping a fluid, such as crude oil, from a well. The invention relates in particular to a fluid driven downhole apparatus for pumping well fluids to the surface and to a method of using the apparatus.
When the formation drive pressure diminishes to a point where insufficient liquid flows from the formation into a well it may be necessary to install pumping means in the well in order to create an artificial lift. Besides beam or rod pumps and gas lift systems, two main types of downhole pumps, namely submersible electric pumps and hydraulically driven downhole pumps, are currently available.
Electric submersible pumps are generally of the centrifugal type and because the pump has to be accommodated in a relatively small diameter tube, the impellers are of small diameter, resulting in a low head per stage and therefore a large number of stages, and a very long pump. Apart from hydraulic jet pumps, which are rather inefficient, presently available hydraulically driven downhole pumps are generally of the turbine type. These do not require as many stages as the electric type, because they are able to operate at higher speeds. However, they are only suitable for relatively high flow rates. The turbine pump furthermore employs high fluid velocities and narrow fluid passages which gives rise to a high wear rate and thus to frequent maintenance if the produced fluid is sand bearing.